This project aims to determine how chondrocyte maturation plays a role in osteosarcoma (OS) oncogenesis. The long term goals are to advance the diagnosis of OS and to identify novel therapeutic targets for OS treatment. OS is the most common bone cancer in children and young adults. It is also the leading cause of cancer death in this age group. The causes of OS development are unknown. Previous studies have suggested the importance of osteoblast activity in its tumorigenesis, since the characteristic feature of OS is the abnormal bone formation in its tumor tissue and the predominant cell type within OS is osteoblast. However, besides osteoblasts, other cell types such as chondrocytes, adipocytes, and fibroblasts also appear within OS. How these cell types contribute to OS pathogenesis remain poorly understood. Previously, cartilage formation has been reported both in mouse and in human OS tissues (Walkley et al., 2008). This is intriguing, since cartilage formation is an essential step of endochondral bone formation, suggesting its putative role in OS development. In this application, the investigators hypothesize that chondrocyte maturation, a critical stage of cartilage development during endochondral bone formation, plays a role in the initiation and progression of OS, possibly through a mechanism relating to chondrocyte apoptosis. To test above hypothesis, the investigators propose to delineate the correlation of chondrocyte maturation with OS development using established mouse OS models and their own novel transgenic mouse models, as well as human OS tissue samples. Recently, preliminary results and/or pending publishing data from the principal investigator's group have shown that transgenic mice overexpressing Runx2 or P63 in hypertrophic chondrocytes result in delayed or accelerate chondrocyte maturation respectively. Given the important function of Runx2 and P63 genes during bone and cancer formation, these mouse models provide unique resources to genetically determine how chondrocyte maturation affects OS progression in a p53 and pRb deficient OS mouse model (Walkley et al., 2008). Three specific aims are proposed. Aim1 is expected to show that delayed chondrocyte maturation in Col10a1-Runx2 mice will accelerate OS initiation and progression due to increased expression of anti- apoptotic marker genes. Aim2 is expected to show that accelerate chondrocyte maturation in Col10a1- TAP63? mice delays OS development, possibly through a proapoptotic mechanism. Aim 3 focuses on analysis of candidate genes (such as P53, P63, RB1, SOX9, RUNX2, etc.), relating to both OS pathogenesis and chondrocyte maturation, in human OS tissues using tissue-array, qRT-PCR and novel COLD-PCR approaches. Successful completion of these aims will advance our understanding of OS etiology and identify promoters of chondrocyte maturation as novel therapeutic targets for OS.